There are a few similar threads at the moment but I believe that time out is only effective if it is a direct consequence of their actions (eg. necc for safety) but if it is for something unrelated (eg. throwing something) then you should follow up with something related to that action (eg. help to pick it up, removal of thrown thing etc.) so they related what they are doing with what follows. It helps them to establish logic and learn to think for themselves rather than something just being a parents rule with no real reason. At 20 months you can also explain to them why you aren't happy. I use single words eg. safety, dangerous, fragile.

I wouldnt use time out for tantrums because tantrums are a toddlers way of dealing with the overwhelming frustration of wanting to do something, not being able to communicate properly and having little power to do things themselves (that is why tantrums come out more when they get tired). The best thing to do for tantrums is just show them love that they will accept (cuddles, pats or just sitting within view if that is all they will allow) and waiting it out. Lots of cuddles once it is over.

Again I wouldnt use time outs out and about because time outs can seem to the child that a rejection of them rather than just their behaviour and this would be magnified by being out of their usual home environment.
Hope this helps. \'0